The field of the invention is improved silver catalysts for the production of ethylene oxide, their preparation and their use in ethylene oxide processes.
The state of the art of processes for preparing ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen in the presence of supported silver catalysts may be ascertained by reference to U.S. Pat. Nos. 4,012,425; 4,039,561; 4,051,068; and 4,125,480, the disclosures of which are incorporated herein. U.S. Pat. Nos. 3,793,231; 3,962,136; 4,066,575 and 3,875,080 disclose the state of the art of silver catalysts for the production of ethylene oxide, the disclosures of which are incorporated herein.
The production of ethylene oxide by the oxidation of ethylene is carried out in the presence of catalysts containing silver. The catalysts used for the oxidation of ethylene into ethylene oxide essentially are prepared by:
(a) depositing an active silver layer by means of a suspension onto a substrate (suspension method) where mechanical mixing or spraying techniques are employed; or
(b) impregnating a porous substrate material with a silver salt solution (impregnation method); the catalysts then being subjected to post-thermal treatment.
With regard to the so-called "suspension method", usually an aqueous suspension of silver oxide is placed on a ceramic substrate material. A cohesive silver layer of varying thickness, depending in the case, is formed on the substrate and then activated. This layer of silver is quite susceptible to damage, whereby silver dust accumulates throughout the reactor tube and causes an increased pressure difference. This pressure difference leads to a decrease in the gas flow and hence to a drop in reactor output. To avoid this drawback, every effort is made so that the silver oxide particles penetrate as deeply as possible into the substrate pores in order to protect them against mechanical abrasion and thus to increase catalyst life.
According to the so-called "impregnation process", an aqueous solution of a silver salt is used in lieu of a suspension of a silver oxide, and the substrate is dipped into this aqueous solution. After absorption of the silver compound into the substrate pores, the impregnated substrate is dried and post-treated thermally. These catalysts suffer from the pronounced drawback that they allow only a low conversion even though they evince good selectivity.
It is furthermore known to add modifying agents (promoters) to the catalysts. Predominantly the compounds of the alkaline earth metals, alkali metals and rare earth metals are recommended as these promoters. Furthermore the addition of alkali metals, especially the "heavy" alkali metals, potassium, rubidium and cesium are known as disclosed in U.S. Pat. Nos. 3,962,136 and 4,066,575. The reports about the optimal preparation of such catalysts are conflicting. These conflicting results are disclosed with respect to selectivity and extent of conversion at a given temperature of reaction depending on whether the promoter is worked into the substrate during a prior process step, during the deposition of the silver compound or after the preparation of the primary silver catalyst.
An especially advantageous process for preparing silver catalysts for the production of ethylene oxide is disclosed in U.S. Pat. No. 3,793,231, and this process is termed herein as a "mixed process". According to the "mixed process" use is made of an aqueous silver oxide suspension wherein a given part of the silver is present as a silver salt. This process achieves both the advantages of the above mentioned impregnation method and those of the suspension method, without incurring the drawbacks of either.